Circuits for measuring the level of RF signals are well known and the earlier of such circuits are of the analog type that utilize two separate stages in which the first stage comprises one or more diodes for modulating and detecting the RF signals and one or more RF amplifiers for amplifying the detected signal. The second stage comprises an analog-to-digital converter. Although the analog circuits serve their intended purpose and advantageously achieve a dynamic range between 30 db to about 70 db, the circuits are somewhat difficult to implement because some of the related RF amplifiers have to be properly aligned; i.e., preset to the RF signals being measured and detected. Not only is the alignment difficult but it also limits the bandwidth of the RF signals that can be measured. Furthermore, the recovery time; i.e., the time for a circuit to recover from momentary overdrives of applied RF signals, are relatively long having a duration of between 300 to 1,000 nanoseconds (nS) primarily due to the inherent operation of the analog circuit components.
Circuit arrangements having built-in analog-to-digital converters that measure and provide a digital representation of applied RF signals are known in the art. One such known circuit utilizes a chain of RF amplifiers that are interconnected to each other by a signal path to which are coupled directional couplers each of which couples a portion of the applied RF signals when such RF signals reach a predetermined power level. The coupled signal from each directional coupler then passes through a diode detector, an amplifier typically being a video amplifier, and to a parallel arrangement of an analog-to-digital converter and a comparator circuit. The RF signals are amplified by the chain of the RF amplifiers each having a designated level limiting its peak amplitude output. The limited RF amplifiers are arranged wherein the lowermost or first RF amplifier receives the RF signals and then the RF signals are increasingly amplified by subsequent RF amplifiers until the RF signals are developed at the uppermost RF amplifier. Commonly, a low level RF input signal is amplified by the chain of RF amplifiers and then first detected by a diode detector on the output stage of the uppermost amplifier. Commonly during such operation, increases in the RF input power level cause the uppermost amplifier to be driven into its limiting condition. However, just before such limitation occurs, the directional coupler and its diode detector on the output stage of the prior amplifier receives a sufficient power level of the RF signals to initiate its operation as well as the operation of its analog-to-digital converter. Similarly, just before this prior RF amplifier goes into its limiting condition, the next or lower prior detector located on the output stage of this next prior RF amplifier receives a sufficient power level of the RF signals to initiate its operation. This chain like reaction continues until the detector at the input stage of the lowermost or first RF amplifier receives a sufficient power level of the RF signals so that its related analog-to-digital converter provides a digital representation on the received RF signals. This prior circuit arrangement serves well its intended purpose, but suffers from the drawback that it lacks a monotonic operation. More particularly, increases in the power level of the applied RF signals do not necessarily cause a smooth increase in the output digital representation. This non-monolithic operation is primarily caused by the use of comparator circuits that are used to switch between the different RF amplifiers of the sequential circuit arrangement. More particularly, the comparator circuits, in cooperation with switching logic, are used to determine which analog-to-digital converter of which respective RF amplifier is to supply the digital output representative of the applied RF signals. Each comparator circuit uses a predetermined threshold value to determine the activation of the analog-to-digital converters. This threshold value is dependent upon fixed voltages that fail to take into account the variations of the RF amplifier's gain due to frequency or temperature changes. Because of this fixed threshold value, the comparator circuit erroneously allows switching between the different chained RF amplifiers which, in turn, causes an inaccurate measurement of the level of the RF signals. It is desired that means be provided that are not susceptible to variations in the components of the circuit that it controls so as to allow for a more accurate measurement of the power level of RF signals.
It is, therefore, a primary object of the present invention to provide a circuit arrangement for measuring the power level of the RF signals that is not susceptible to inaccuracies caused by variations in the circuit elements while still providing a relatively low recovery time.
It is another object of the present invention to provide for an accurate measurement of the power level of transient or continuous RF signals over a wide dynamic power range and for generating a digital representation of such measurement.
It is a further object of the present invention to provide a circuit arrangement generating a digital representation indicative of the power level of the various RF signals while still providing a monotonic operation that follows any increases in the level of the RF signals and causes a corresponding smooth increase in the digital representation.
It is a further object of the present invention to provide means so that the applied RF signals are accurately sampled; i.e., sampled at their half-power points.
Other objects of the present invention, as well as the advantages thereof over exisiting and prior art forms, which will be apparent in view of the following detailed description are accomplished by means hereinafter described and claimed.